Although applicable in principle to any micromechanical components, the present invention and the problem underlying it are explained with reference to a micromechanical rotation rate sensor using the manufacturing technology of surface micromechanics, as described in German Patent Application No. 195 30 736.
FIG. 6 shows a conventional micromechanical rotation rate sensor. FIGS. 7-13 show conventional steps for manufacturing the micromechanical rotation rate sensor shown in FIG. 6.
This rotation rate sensor is constructed from a three-layer system, usually from an SOI (silicon-on-insulator) system, in particular a silicon-insulator-silicon system.
It has a first layer 1 made of lightly doped silicon, a second layer 2 overlying it made of silicon dioxide, and a third layer 3 overlying that made of heavily doped silicon, which can also be polycrystalline.
FIG. 6 shows a frame 8 in which a vibratory mass 5 is arranged via lands 9 in a manner allowing vibration. Vibratory mass 5 is excited via drive means 21 (Lorentz forces) to vibrate linearly in the arrow direction. A conventional acceleration sensor 6 having a usual comb structure is arranged on vibratory mass 5. Acceleration sensor 6 is applied in the detection direction, perpendicular to the vibration direction of vibratory mass 5.
FIG. 6 further shows conductor paths 4 which proceed from retaining beams 18, 19 and the deflectable mass 7 of acceleration sensor 6 and pass via lands 9 to frame 8. Conductor paths 4 are patterned out of third layer 3, and in frame 8 are electrically insulated by recesses 10 from third layer 3 of frame 8.
Lands 9 are patterned both out of the upper third layer 3 and out the lower first layer 1. Conductor paths 4 arranged on frame 8 transition directly into lands 9, so that direct electrical contacting of the acceleration sensor is achieved through them.
On vibratory mass 5, recesses 10 are also provided in the outlet opening region of lands 9 as insulating trenches in upper layer 3, so that here again electrical insulation is guaranteed for the signals which pass via the lands from and to vibratory mass 5. Conductor paths 4 end in connector pads 20 from which the signals are transmitted to an electronic analysis system.
The manufacturing process of the rotation rate sensor shown in FIG. 6 will be explained below in more detail with reference to FIGS. 7-13.
On third layer 3, preferably aluminum metal strips 24 are formed in the regions of conductor paths 4 by vacuum evaporation or sputtering and subsequent patterning. A first cover layer 11 made of silicon dioxide and a second cover layer 12 made of plasma nitride are applied on the back side of first layer 1, and a third cover layer 14 made of silicon dioxide is applied on the front side of third layer 3 equipped with aluminum metal strips 24.
Third cover layer 14 is then patterned in accordance with comb structure 13 of the acceleration sensor and the shape of conductor paths 4 and lands 9. A fourth cover layer 16 in the form of a photoresist is applied onto the patterned third cover layer 14. This fourth cover layer 16 is removed in the region of lands 9. The etching mask here is the oxide; the resist protects only the acceleration sensor.
After corresponding patterning of first and second cover layers 11, 12, first layer 1 inside frame 8 is then etched away to a predefined thickness of typically 100 .mu.m, and covered with a passivation layer.
In an etching process, trenches for which the patterned third cover layer 14 and fourth cover layer 16 serve as etching mask are then introduced on the front side. These trenches are etched in until second and third layers 2, 3 have been completely penetrated and a portion of first layer 1 has been removed, specifically until the remaining thickness corresponds approximately to the thickness of layer 3. Fourth cover layer 16 is then removed, and the etching process is continued until first layer 1 has also been completely penetrated. The patterned third cover layer 14 now acts as an etching mask in the acceleration sensor region, which contains the patterns for lands 9, comb structures 13, and conductor paths 4. The etching process is configured so that only first and third layers 1, 3, but not second layer 2 and passivation layer 17, are etched, preferably anisotropically in SF.sub.6 plasma.
Altogether, etching is continued until the trenches for lands 9 reach passivation layer 17, and the trenches for comb structures 13 and conductor paths 4 reach second layer 2.
Second layer 2 is then etched out beneath comb structures 13, and simultaneously lands 9 are etched out of second layer 2, the etching medium preferably used for this purpose is HF gas, as described in German Patent No. 43 17 274.
Passivation layer 17 is also etched away, in order to produce the rotation layer shown in FIG. 6.
The problem on which the present invention is based is in general the fact that the underetching labeled U in FIG. 13 is desired and necessary for the function of acceleration sensor 6 in the region of comb structures 13 with their typically 11-.mu.m thick and 5-.mu.m wide polycrystalline silicon, but is extremely troublesome in the region of the supply leads, i.e. of lands 9 with conductor paths 4.
In the finished product these supply leads, which have a mechanical spring function especially in the region between frame 8 and vibratory mass 5, are typically made up of a layer sequence of 65-.mu.m thick monocrystalline silicon as first layer 1, 2.2-.mu.m thick silicon dioxide as second layer 2, 1-.mu.m thick polycrystalline silicon as third layer 3, and an aluminum layer 1.3 .mu.m thick.
Because of the exposed location of comb structures 13, the HF gas attacks second layer 2 particularly vigorously in the region of the supply leads. If the second layer is excessively underetched or even removed in this region, the resulting component is unusable, since the mechanical connection and electrical insulation between first and third layers 1, 3 is lost. This phenomenon leads to poor yields for the existing manufacturing process explained above.
One usual approach to limiting underetching is to adjust the geometry of the rotation rate sensor such that the silicon dioxide of second layer 2 that is to be etched requires only 2.5 .mu.m of underetching, the supply lead oxide being 21 .mu.m wide. This allowance is more than compensated for, however, by the preferential etching of the supply lead oxide, so that poor yields still occur.
Another possible approach would be to replace the supply lead oxide with a slower-etching second layer 2, but it has been found that the dielectric layers commonly used in semiconductor technology, such as nitride, etc., have too little resistance to the HF gas.
It is therefore desirable to protect the supply lead oxide more effectively in a relatively simple process.